1. Field of the Invention
This invention relates to a closed user group facility, and more particularly to a closed user group facility in which those terminals accomodated in a switching network which are already registered mutually as distant terminals, form closed user groups; and the switching network ensures smooth transmission between the terminals in the closed user group, and at the same time, prevents the transmission in the closed user group from being disturbed by terminals belonging to other groups.
2. Description of the Prior Art
Main methods in a conventional closed user group facility in switching networks are the identify code method and the group code method. In the identify code method, the switching office having received a request to send from an originating terminal sends to a terminating terminal an identify code indicative of a closed user group. The terminating terminal decides whether or not the identify code is coincident with that of the closed user group to which the terminating terminal belongs, and receives data from the originating terminal only in the case of coincidence.
This method requires the provision of the identify code deciding function in each terminal, and hence has a demerit that the terminal is costly. Further, whether or not a request for the closed user group service is proper is decided by deciding the identify code sent to the terminating terminal, and there are some cases where a subscriber line and an intermediate line are held ineffective, so that the transmission efficiency of the network is lowered.
In the group code method, upon receipt of a request to send from a terminal, a switching office of a network discriminates the group codes of the closed user groups of the originating terminal from the subscriber data of the switching office, and sends out the group code to a switching office having accomodated therein the terminating terminal. The terminating office decides whether or not the group code of the closed user group to which the terminating terminal belongs is coincident with one of the group codes received from the originating office, and only in the case of coincidence, permits transmission between the originating and terminating terminals.
This method requires the switching network to collectively manage all the group codes of the network for maintenance and management of the closed user groups, and has a defect in the cost for maintenance and management of the network attendant with the numbering plan of the group code and entry and withdrawal of the terminal use in or withdrawal from the group. Further, the originating office is required to send to the terminating terminal all of the group codes of the originating terminal, and the terminating office is required to match its group code with all the group codes of the originating terminal at each request for transmission. This leads to a defect that a load to the switching equipment increases. Moreover, it is necessary to inquire of the terminating office about the appropriateness of the request for the closed user group service from the originating terminal. Granting that ineffective holding of the subscriber line and intermediate line does not occur as many times as in the abovesaid identify code method, the transmission efficiency of the network is still low.